This grant proposes preclinical studies to improve understanding of the cellular basis of vascular repair in the adult lung by a blood-derived cell population. We have identified a novel population of circulating endothelial precursors (CEPs) and discovered their potential to repair capillary segments in the murine lung by a previously undescribed angiogenic process. Based on preliminary data, the studies supported by this grant will be guided by three principal hypotheses: (i) in lung vascular injury CEPs fuse to endothelial cells, acquire an endothelial-like phenotype, and engraft into existing capillaries to repair their structure and function;(ii) BMDCs mobilized into the circulation and recruited to the injured lung make a major contribution to capillary repair by CEPs;and (iii) VEGF and SDF-11 signaling mediate the mobilization and recruitment of BMDCs to the injured lung and capillary repair by CEPs. State-of-the-art high resolution and fluorescence imaging approaches (using immunogold labels and quantum dots), and other quantitative approaches, i.e., flow cytometry analyses, will be used. The studies proposed will take advantage of genetically modified mice and a well-characterized model of capillary repair by CEPs in the lung injured by breathing high oxygen. Repair in acute lung injury and in the chronically injured lung - in which capillary networks are lost and then restored - will both be studied. Specifically, we aim to characterize the kinetics and phenotype of murine CEPs as they restore pulmonary capillaries (AIM 1);demonstrate in structural and functional studies, that include myelo-suppression and infusion of BMDCs sub-sets, a causal relationship between BMDCs and capillary repair by CEPs (AIM 2);and demonstrate that selective blockade of VEGF/VEGF-R2 and/or of SDF-11/CXCR4 signaling impair the mobilization and recruitment of BMDCs to the injured lung and capillary repair by CEPs (AIM 3). By understanding the role of CEPs in the salvage of existing capillary networks, the proposed studies should promote the development of cell-based treatment strategies for the acutely injured lung as well as the lung with established capillary loss. PUBLIC HEALTH RELEVANCE: In life-threatening diseases such as the Pulmonary Hypertensions, or following acute lung injury leading to the development of a severe form of Acute Respiratory Distress, vascular injury destroys small blood vessels throughout the lung. Studies are designed to improve understanding of a newly identified process of spontaneous repair of these blood vessels by cells circulating in blood. Further understanding of how such circulating cells salvage existing blood vessels, or trigger growth of new ones, will promote the development of cell-based therapies to restore damaged vascular networks in the lung.